(1) Field of the Invention
The invention relates generally to data storage in semiconductor memory devices and more particularly, to memory devices equipped with sense amplifiers or additional read-out electronics such as Read Only Memory ROM, Flash memory, Random Access Memory (RAM), and also Magnetic RAM (MRAM). These memories will in the following altogether be subsumed for brevity under the designation of Sense Electronics Endowed (SEE) memories. Even more particularly this invention is relating to the timing schedule during memory read of a memory read/enable transition signal in an SEE memory device with reduced power consumption.
(2) Description of the Prior Art
Microprocessor systems can be found nowadays working in many devices, such as Personal Computers (PCs) especially modern portable notebook computers, in Personal Data Assistants (PDAs), mobile phones, navigation systems—mostly also used as portable devices, but also in many household appliances, in automobiles etc. and they all have Central Processing Units (CPUs) which need some sort of Random Access Memory (RAM) for their primary workspace (in RAM the code and data for the CPU are stored) usually implemented as semiconductor memory, wherein the contents of each byte can be directly and randomly accessed. Other types of memory chips, including ROMs and PROMs have this property as well, but only RAM chips are economically priced however they require power to maintain their content. The most common type of computer memory in current solid-state memory technology for main memory storage, which usually uses one transistor and a storage capacitor to represent a bit, is called Dynamic RAM (DRAM). Therein the capacitors must be energized hundreds of times per second in order to maintain the charges, representing the stored information as data. A data bus system is used for moving the information in and out of the RAM storage and an address bus addresses the storage location of the information data within the RAM. The RAM is usually organized in a grid or matrix configuration, where each bit is stored in its own data cell and each row and column has its own address. Another implementation called Static RAM (SRAM) is a type of RAM that holds data without need to refresh the stored content. An SRAM bit is made up of 4 to 8 transistors and is therefore very fast, with access times in the 10 to 30-nanosecond range but also power dissipating and expensive to produce. In comparison, DRAM only uses one transistor per memory cell and has access times, which are usually above 30 ns. SRAM does not require any refreshing operation and is easily handled, but is more expensive than DRAM and has a smaller capacity than DRAM comparing the same chip area. Because of these properties, SRAM is used to create a CPU's speed-sensitive cache, while DRAM is used for the larger system main storage RAM space. The memory internal operations during read, write, and refresh transactions are governed by a number of control signals allowing to strobe or clock addresses and data in and out, and to partially select, enable or inhibit these operations. All these operations are more or less power consuming, which leads especially with portable devices to reduced power-on times, as these devices are dependent from the energy capacity stored in their battery. It is therefore important to reduce power consumption during operation of the device, and the operation of the memory especially.
In the prior art, there are different technical approaches for achieving the goal of a reduction of power consumption. However these approaches use often solutions, which are somewhat technically complex and therefore also expensive in production. It would be advantageous to reduce the expenses in both areas. This is achieved by using a transitional de-activation process for parts of the memory operation, when not in full service. Using the intrinsic advantages of that solution—as described later on in every detail—the circuit of the invention is realized with standard CMOS technology at low cost.
Preferred prior art realizations are implementing such related memory circuits in single chip or multiple chip solutions as integrated circuits. The permanent high power requirement and therefore high system costs are the main disadvantages of these prior art solutions. It is therefore a challenge for the designer of such devices and circuits to achieve a high-quality but also low-cost solution.
Several prior art inventions referring to such solutions describe related methods, devices and circuits, and there are also several such solutions available with various patents referring to comparable approaches, out of which some are listed in the following:
U.S. Pat. No. 6,008,821 (to Bright et al.) describes an embedded frame buffer system and synchronization method, wherein a multiple embedded memory frame buffer system includes a master graphics subsystem and a plurality of slave graphics subsystems. Each subsystem includes a frame buffer and a color palette for decompressing data in the frame buffer. The master subsystem further includes a digital to analog converter coupled to receive the decompressed digital data from the palette of each subsystem and outputting analog versions of the digital data to an output device. The system further includes a timing system for determining which outputs of the subsystems are to be converted by the digital to analog converter at a given time. A method of synchronization of embedded frame buffers for data transfer through a single output includes the steps of generating a first clock signal and a second clock signal in a master embedded frame buffer, sending the first and second clock signals to a slave embedded frame buffer and delaying the second clock signal to be in phase with a third clock signal generated by a graphics controller such that no data is lost when transferring data from the master and slave embedded frame buffers.
U.S. Patent Application 2003/0081487 (to Mizugaki) discloses a semiconductor memory device whereby the technique of the present invention sets a time period of a level H between a rise and a fall of an ATD signal (that is, a pulse width of the ATD signal) to be not shorter than a preset allowable address skew range and not longer than a time period between a timing of a rise of the ATD signal, at which the refreshing operation starts, and conclusion of the refreshing operation. This arrangement ensures generation of an appropriate ATD signal even when an address skew occurs in an externally given address.
U.S. Patent Application 2004/0044921 (to Sakaino et al.) describes an integrated circuit device externally connected with a peripheral device operated by a first clock signal. The integrated circuit device includes a CPU having information on the frequency of the first clock signal, a clock generator generating a second clock signal for operating the CPU and outputting third clock signals obtained from the second clock signal and a clock halt portion receiving the third clock signals and selectively outputting only one of the third clock signals according to the information. The integrated circuit device further includes a timer activated only when receiving the one of the third clock signals and converting the frequency of the received clock signal for output and a clock synchronization serial port receiving the clock signal outputted from the timer and one of the other third clock signals, and supplying either one of the received clock signals to the peripheral device according to the information
The basic RAM circuit is shown in FIG. 1 prior art in form of a modified circuit diagram (i.e. with graphical representation of the memory array as grid layout) with a storage (RAM) cell 10 as central component, wherein the information is stored as a single bit, in this case. Arranging these data storage cells 10 in form of a rectangular grid unfolds the core bit/word (X/Y) organized memory array element, with horizontal rows 12 and vertical columns 11 spanning a storage matrix 15 with Cartesian X and Y coordinates identifying the X/Y data cell location 10 and in such a way setting up the main storage area organized in bits (X) and words (Y). In technical terms the columns are designed as bit lines 11 and the rows as word lines 12, the storage (RAM) cells 10 can be implemented as single transistor-capacitor DRAM or multiple transistor SRAM cells, or even as formerly used magnetic cores or MRAM (Magnetic RAM) devices of late. This memory array is now addressed through the address bus system from the processor CPU with addresses made up of a Row Address 22 part and a Column Address 24 part, the Row Address 22 part being decoded in a Row Decoder 21 and the Column Address 24 part being decoded in the address part of a Column Decoder 25. The Row Decoder 21 is then activating the according word line 12, whereas the address part of the Column Decoder 25 activates the according bit line 11. Depending on the operation to be performed a Write, Read or Refresh cycle for the selected storage (RAM) cell 10 is then executed. Therefore Read/Write Circuits 40 are activated, performing the according bitwise data operations with the help of Sense Amplifiers 30 and acting on the particular storage (RAM) cells 10. The relevant data are delivered via said Column Decoder 25 too, having additionally a data part, connected to the Input/Output data bus system of the CPU. These data are therefore written into or read from the main memory array in parallel with the help of said Column Decoder 25 connected to said Read/Write Circuits 40 and these further connected on their part to said Sense Amplifiers 30 writing or reading the contents of the connected storage (RAM) cells 10. Refresh operations are essentially made up of a combination of Read/Write operations. The length of the address as shown in the figure is k bits and depends on the size of the addressable memory—defining also the bus width of the address bus, and the length of the data word as shown in the figure is M bits and depends on the CPU type—determining also the bus width of the data I/O bus.
In FIG. 2A prior art a timing diagram shows the time response of the relevant signals during a normal memory read cycle, describing the two critical points of time named ADDRESS and DATA (shown respectively as 1 and 2 encircled) of the “System Clock” signal 19 used as clock signal, where the two main actions of a normal memory read cycle are triggered. The rising edge at point of time ADDRESS (1 encircled) of the “System Clock” signal 19 thereby triggers the addressing of the memory, which indicates that a valid address is being applied now via its address bus shown with signal “Mem Address” 29 and being kept valid until point of time DATA (2 encircled), where the memory data for this valid address can now be read out as signal “Mem Data out” 49. For technical reasons accounting and providing for eventually occurring signal transit times between point of time ADDRESS and the final validity of the complete address made up of k bits a delayed sampling at point of time DATA is necessary, where all M bits of the data word are considered to be valid. This effect is called address skewing, by the way. Now either during this whole duration of time or more often during one half of that time (corresponding to one half of the “System Clock” period) a “Mem Read/enable” 39 signal is set, allowing or better forcing the Sense Amplifiers 30 and the Read/Write Circuits 40 to operate. A substantial part of the power consumption i.e. power loss of the memory is due to that fact. Aiming now our attention to FIG. 2A prior art again this course of events is not only proceeding during a real read operation, it has to be repeatedly handled also during the many refresh cycles as adumbrated with the dashed lines in the figure, and finding a procedure enhancing and ameliorating this performance is therefore a challenge for the designer of such devices and circuits, thus achieving a high-quality and also low-cost solution, avoiding these drawbacks.
Although these patents and papers describe circuits and/or methods close to the field of the invention they differ in essential features from the method, the system and especially the circuit introduced here.